When protective relays are used within electrical power transmission systems in an overload protection capacity, the relay must rapidly respond without delay to insure that the associated transmission equipment is unharmed.
State of the art protective relays include a circuit to overdrive a conventional electromagnetic relay by using a higher voltage than the relay coil design specifies and then limiting the current either by an electronic current source in the coil circuit or by shorting a series resistor in the coil circuit and using a semiconductor switch such as a thyristor to decrease the relay overall response time.
A second approach includes a pair of relay contacts one of which is normally closed to provide an initial high current path into the relay coil. Once the relay contacts begin to move, the normally closed contacts open, removing the higher current from the coil. A hold-in series resistor provides continued drive after the relay closes.
A further approach uses thyristors in place of the relay contacts as the switching devices. Turn-on time for thyristors can be very fast and state-of-the-art thyristors can handle large currents instantaneously. However, the thyristors must be sized to limit power loss associated with the large quiescent currents within electrical power transmission systems and must be polarized with respect to the direction of current flow.
U.S. Pat. No. 5,079,457 entitled "Dual Solid State Relay" describes the use of solid state relays that employ both Triacs and SCRs in protective relay applications.
U.S. Pat. No. 5,162,682 entitled "Solid State Relay Employing Triacs and a Plurality of Snubber Circuits" discloses the use of an optical coupler combined with a triac and a snubber circuit to protect electrical equipment.
U.S. Pat. No. 5,338,991 entitled "High Power Solid State Relay with Input Presence and Polarity Indication" describes the application of an optical coupler with a solid state Darlington circuit to provide solid state relay function.
Such solid state relays, however, are generally expensive, do not provide adequate ohmic isolation and require particular attention to polarity during installation within the protected circuit.
Recent approaches to the combination of custom relay contacts with custom semiconductor switches for specific applications are found in U.S. Pat. No. 4,992,904 entitled "Hybrid Contactor for DC Airframe Power Supply" and U.S. Pat. No. 5,536,980 entitled "High Voltage High Current Switching Apparatus".
In view of the excellent properties of conventional protective relays employing standard coils and contacts to cover a wide range of operating currents, is would be highly advantageous to modify the response time thereof to allow use within those applications requiring immediate contact separation.
One purpose of the invention is to provide a hybrid protective relay having the fast response features of a solid state relay while retaining the low cost and high performance of an electromagnetic protective relay.